Without summarisation and filtering, a network of routers can drown in its own chatter. Every tiny change in a remote part of the network triggers a flood of updates, wasting bandwidth and CPU power. For the 300-410 exam, you need to know how to configure and verify EIGRP summarisation and filtering to build stable, efficient networks.
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A simple way to picture EIGRP Route Summarization and Filtering
A massive public library with thousands of books, spread across dozens of shelves and rooms. The head librarian needs to tell the central catalogue system every time a new book arrives, or a book is moved, or a book is removed. At first, this is manageable, but as the library grows, the updates become a flood. Each tiny change — a single book shifted one shelf over — generates a message to the central system. The system becomes sluggish, overwhelmed by these tiny, individual updates.
The librarian decides to use a smarter approach: instead of reporting every single book, she reports summaries. For the entire 'History' section, she sends one update: 'All history books are in aisle 5, shelves A through F.' She filters out the noise. If a single history book is moved to a different shelf but stays within that range, no update is sent. This is summarisation. She also decides to block certain reports entirely. The library decides it will not accept books about 'gardening in Antarctica' because they are never requested, so the librarian tells the central system to ignore any messages about those topics. This is filtering. The central system now has fewer updates to process, it runs faster, and the network of information between branches stays clean and efficient.
Welcome to the world of interior routing protocols. EIGRP, which stands for Enhanced Interior Gateway Routing Protocol, is a Cisco proprietary protocol. Think of it as a smart, efficient mailman inside a large office building. EIGRP routers talk to each other, sharing information about how to reach different parts of the network.
But here is the problem: in a large network, there can be thousands of routes. Every time a router learns about a new route, or a route goes down, it tells all its neighbours. This can create a lot of traffic, which we call routing updates. If every single route is shared individually, the network can slow down. This is where route summarisation and filtering come in. They are two separate, but related, tools to manage this information flood.
Route Summarisation
Imagine your network is a city. You have a street called 'Main Street' and it has two branches: 'Main Street North' and 'Main Street South'. Main Street North has addresses 10.1.1.0 through 10.1.1.255. Main Street South has 10.1.2.0 through 10.1.2.255. Instead of telling everyone in the city about every single house number, you can summarise. You tell them, 'For anything starting with 10.1.x.x, send it to Main Street.' This is a summarised route.
In EIGRP, summarisation is configured on an interface. You tell the router, 'On this interface, do not advertise the specific subnets you know about. Instead, advertise this one summary route.' The router calculates the summary route based on the individual routes in its routing table. This reduces the size of the routing table on other routers and reduces the number of updates sent.
Automatic Summarisation (classful): This is an older EIGRP feature. It summarises routes to their classful boundary. For example, 10.1.1.0/24 and 10.1.2.0/24 are automatically advertised as 10.0.0.0/8. Because this is often too broad and can cause routing loops, it is disabled by default in modern networks.
Manual Summarisation (on interface): This is what you will configure on the exam. You choose a specific prefix and subnet mask to advertise. It gives you precise control.
Route Filtering
Filtering is about deciding what information to keep and what to throw away. Imagine you are a security guard at the building entrance. You have a list of allowed visitors. If someone is not on the list, you turn them away. Route filtering in EIGRP works the same way. You can create a list of routes you want to accept or deny. This is done using something called a distribute list.
A distribute list is applied to inbound updates (routes coming in) or outbound updates (routes going out). It uses an access list (ACL) or a prefix list to match routes. A prefix list is like a very precise filter for IP addresses. It is more powerful and easier to read than an ACL for routing purposes.
For example, you might have a policy that says: 'We do not want any routes from the 192.168.0.0/16 range entering this part of the network.' You configure a distribute list with a prefix list that denies that range, and apply it inbound on the interface. Now, the router will ignore any updates about those routes.
Why both exist
Summarisation reduces the quantity of information by grouping routes. Filtering reduces the quantity by entirely blocking the information you do not want. You can use them together. For instance, summarise the 'North' block of routes, then filter out the 'South' block entirely. On the exam, you need to know the commands to create prefix lists, apply them with distribute lists, and summarise routes on interfaces. The key is understanding that summarisation is a 'lumping' tool and filtering is a 'blocking' tool.
Identify the subnets to summarise
First, list all the specific subnet prefixes that exist in the part of the network you want to summarise. You need to find a single summary address (a block of addresses) that covers all of them. For example, subnets 10.1.1.0/24 and 10.1.2.0/24 can be summarised as 10.1.0.0/22.
Configure manual summarisation on the correct interface
Access the interface configuration mode (e.g., 'interface GigabitEthernet0/0'). Issue the command 'ip summary-address eigrp 1 10.1.0.0 255.255.252.0'. The router will immediately advertise the summary route to neighbours out of that interface. The specific routes are suppressed from being advertised individually out of that interface.
Verify the summary route is in the routing table
Use the command 'show ip route' on the summarising router. You will see the summary route as a directly connected route to Null0. On the neighbouring routers, use 'show ip route' to confirm the summary route appears as a learned EIGRP route. This confirms the summarisation is working.
Create a prefix list for filtering
Use 'ip prefix-list MY_FILTER seq 5 deny 10.1.1.0/24' then 'ip prefix-list MY_FILTER seq 10 permit 0.0.0.0/0 le 32'. This prefix list denies a specific subnet but permits everything else. Prefix lists are matched in order by sequence number, and the first match is applied.
Apply the distribute list to filter routes
Enter the EIGRP configuration mode with 'router eigrp 1'. Use 'distribute-list prefix MY_FILTER in' (or 'out') to apply the filter. The direction depends on whether you want to block routes entering or leaving the router. Verify with 'show ip eigrp interfaces' and checking the filter field.
An IT professional, Sarah, is responsible for a company with three branch offices connected to a central headquarters. Each branch office has its own subnets: 10.10.0.0/24, 10.10.1.0/24, and 10.10.2.0/24. The link between the branches and HQ is a slow, expensive MPLS link.
Without any optimisation, every time a branch office changes a route, it sends an update to HQ. The HQ router then sends updates to all other branches. The slow link becomes saturated with routing updates, causing delays for real user traffic like email and file transfers. Sarah decides to configure EIGRP summarisation.
Step-by-step scenario:
Sarah logs into the branch router via SSH (secure remote terminal).
She identifies the three subnets used at that branch.
She calculates a summary route: 10.10.0.0/22. This covers all the branch subnets.
On the branch router's interface facing HQ, she configures: ip summary-address eigrp 1 10.10.0.0 255.255.252.0.
Now, instead of sending three separate updates, the branch sends one summary. If a single subnet goes down (e.g., 10.10.1.0/24), the branch router does not send an update to HQ, because the summary route still points to the branch. If all three go down, it removes the summary.
But Sarah also has a security requirement. The company does not want the HR department's subnet (10.20.0.0/24) to be known at the branch offices. She configures route filtering.
Filtering scenario:
Sarah creates a prefix list on the HQ router: ip prefix-list DENY_HR seq 5 deny 10.20.0.0/24 then ip prefix-list DENY_HR seq 10 permit 0.0.0.0/0 le 32.
She applies this distribute list inbound on the HQ interface facing the branch: router eigrp 1 then distribute-list prefix DENY_HR in.
Now, when the HQ router receives an update about the HR subnet, it checks the prefix list. It matches the deny, so the route is dropped and never entered into the routing table.
Because of summarisation, the slow link is not flooded with tiny updates. Because of filtering, sensitive routes stay isolated. The network is faster and more secure. Sarah's monitoring system shows a 60% reduction in routing traffic.
The 300-410 exam tests your ability to configure and verify EIGRP route summarisation and filtering. You will see both configuration scenarios and troubleshooting scenarios. The exam loves to test the subtle differences between automatic and manual summarisation, and the exact syntax for prefix lists and distribute lists.
Key concepts tested: - Understanding that manual summarisation overrides automatic summarisation. - Knowing that summarisation is configured per interface, not globally under the router process. - The effect of summarisation on the routing table: it creates a local route with a null0 next-hop to prevent routing loops. - How distribute lists work with prefix lists and ACLs. - The difference between inbound and outbound filtering.
Common trap patterns: - A question might show a router with automatic summarisation enabled and then ask you to configure a manual summarisation. The trap is that manual summarisation must be configured on the interface, and it will create a more specific summary than the classful one. Many candidates forget to disable auto-summary or apply the command to the wrong interface. - Another trap: a distribute list applied in the wrong direction. For filtering out routes from a specific subnet to propagate, the list must be applied outbound on the router that knows about those routes. Applying it inbound on the receiving router will block the routes from entering the table, but the update still travelled across the link, wasting bandwidth. - Prefix list matching: a question may ask you to match all routes except a specific one. The correct prefix list will have a 'deny' for the specific route, followed by a 'permit 0.0.0.0/0 le 32' to allow everything else. Leaving out the final permit will block all routes.
Key definitions to memorise:
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- Null0 route: The route created by summarisation to drop packets that match the summary but not a more specific route. It prevents loops.
- Distribute list: The command used to filter routes in EIGRP.
- Prefix list: The modern way to match routes for filtering, more precise than ACLs.
- Manual summarisation: The command ip summary-address eigrp <as-number> <prefix> <mask>.
- Variance: Not directly related, but often compared with summarisation in exam questions. Know it is for unequal-cost load balancing.
The exam will present a topology diagram and ask you to choose the correct summarisation or filtering command. Practice identifying which router and which interface needs the command. Also, memorise the output of show ip route and show ip eigrp topology to verify the summary or filter is working.
EIGRP route summarisation reduces the number of routing updates by advertising a single summary route that covers a group of more specific subnets.
Manual summarisation is configured on the interface level using the command 'ip summary-address eigrp AS_NUMBER PREFIX MASK' and creates a Null0 route to prevent loops.
Route filtering in EIGRP is implemented using a 'distribute-list' command, which can reference either an access list or a prefix list to match routes.
A distribute-list applied in the 'in' direction filters routes before they enter the routing table, while 'out' filters routes before they are advertised to neighbours.
Prefix lists are more efficient and easier to read for routing filter applications compared to standard or extended access lists.
Always end a prefix list with a 'permit 0.0.0.0/0 le 32' statement to avoid the implicit deny that blocks all other routes.
These come up on the exam all the time. Here's how to tell them apart.
Automatic Summarisation
Enabled globally under router eigrp with 'auto-summary'.
Summarises to classful boundaries (e.g., 10.0.0.0/8).
Can cause routing loops in discontiguous networks.
Manual Summarisation
Configured per interface with 'ip summary-address' command.
Allows exact control over summary prefix and mask.
Creates a Null0 route for loop prevention.
Distribute List Inbound
Filters routes before they enter the local routing table.
Saves memory on the local router.
Does not reduce updates on the link from the neighbour.
Distribute List Outbound
Filters routes before they are sent to the neighbour.
Saves bandwidth on the link.
Neighbour never sees the filtered routes.
Prefix List
Matches both IP address and subnet mask (ge/le operators).
More efficient for large routing policies.
Uses a single line with exact prefix and mask.
Access List (Standard)
Matches only IP source address (not mask).
Can become a large list of individual entries.
Older, less intuitive for route filtering.
Mistake
Route summarisation in EIGRP only works with automatic summarisation enabled.
Correct
Manual summarisation works independently of automatic summarisation. You can configure it even if auto-summary is disabled, which is the typical modern practice.
Newer candidates learn about auto-summary first as a basic feature, and mistakenly assume manual summarisation is an extension or requires it to be on.
Mistake
Applying a distribute-list in the inbound direction is the same as applying it outbound because both block the route from being used.
Correct
Inbound filtering blocks the route from entering the routing table on the local router. Outbound filtering blocks the route from being advertised to a neighbour. The traffic still travelled over the link for inbound filtering, but not for outbound.
The end result (route not in table) looks the same from a high level, so beginners overlook the impact on link utilisation and the order of operations.
Mistake
EIGRP summarisation creates a summary route that points to the specific next-hop of one of the subnets being summarised.
Correct
EIGRP creates a summary route pointing to a Null0 interface on the router that is performing the summarisation. This is to prevent routing loops if the summary route is more specific than a default route.
Intuitively, you would think the summary should point to a real neighbour, but the protocol uses a black-hole route for loop prevention. This is counter-intuitive without studying the logic.
Mistake
A prefix list with a 'deny' entry will block all routes that match, and the router will then advertise nothing else.
Correct
Every prefix list ends with an implicit deny rule. If you only have a 'deny' entry, all routes are denied. You must add a 'permit any' statement at the end to allow other routes through.
The default behaviour of ACLs and prefix lists is to deny everything at the end. Beginners forget to add the permit statement, causing a black hole.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
Summarisation groups multiple specific routes into a single broader route to reduce the number of updates. Filtering blocks specific routes entirely from being advertised or received, using a distribute list.
It is disabled by default on modern Cisco IOS devices because it can cause routing loops and suboptimal routing. Manual summarisation on interfaces is preferred.
Use 'show ip eigrp interfaces detail' to see the filter applied on each interface. Also 'show ip protocols' shows a summary of all routing protocols and any filters.
The Null0 route prevents routing loops. If the router receives a packet matching the summary route but not a more specific route, the packet is dropped instead of being forwarded back out to a neighbour.
Yes, you can apply it inbound or outbound. Inbound filters routes before they enter the routing table. Outbound filters routes before they are sent to neighbours. They are applied separately.
You can use either, but prefix lists are recommended for routing filters because they are more precise (they can match subnet mask length) and easier to read. Access lists can match IP addresses but are less efficient for prefix matching.
You've finished EIGRP Route Summarization and Filtering. Continue through the 300-410 study guide to build a complete picture of the exam.
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