BGP: WHAT ARE UPDATE-GROUPS?

Sometimes when you’re looking at a router’s BGP routing table, you might see that a prefix is being advertised to an “update-group”:

Router3#show ip bgp 50.50.50.50
BGP routing table entry for 50.50.50.50/32, version 31
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
 Advertised to update-groups:
 2 3
 Local, (Received from a RR-client)
 69.69.69.253 (metric 2) from 69.69.69.253 (69.69.69.5)
 Origin IGP, metric 0, localpref 100, valid, internal, best

Now, you might think to yourself: wait a second, I didn’t configure any update-groups. Unless I did it when I was drunk? We often do that, don’t we. We go out on the drink with our mates, have a blinding night, come home wasted out of our minds, and do the only logical thing: configure some BGP update-groups, like absolute bloody legends.

Well, I’m happy to tell you that this time you didn’t get up to any DBGP (Drunken Border Gateway Protocol): this happens automatically.

BGP UPDATE-GROUPS

Update-groups are a way for Cisco router to save on resources, by grouping neighbors together that would receive the same updates. These groups are made automatically and dynamically – you don’t need to do anything.

You usually don’t need to worry about what routers are in what groups. But if you do want to know who’s in the update-group, use this command:

Router3#show ip bgp update-group 3
BGP version 4 update-group 3, internal, Address Family: IPv4 Unicast
 BGP Update version : 31/0, messages 0
 4 octets ASN capable
 Route-Reflector Client
 Extended-community attribute sent to this neighbor
 Update messages formatted 12, replicated 32
 Number of NLRIs in the update sent: max 4, min 0
 Minimum time between advertisement runs is 0 seconds
 Has 4 members (* indicates the members currently being sent updates):
 69.69.69.249 69.69.69.250 69.69.69.253 69.69.69.254

One final thing, from this article on Cisco’s website:

When a change to outbound policy occurs, the BGP routing process will automatically recalculate update-group memberships and apply changes by triggering an outbound soft reset after a 1-minute timer expires. This behavior is designed to provide the network operator with time to change the configuration before the soft reset is initiated. You can immediately initiate the outbound soft reset before the timer expires by entering the clear ip bgpip-address soft out command or immediately initiate a hard reset by entering the clear ip bgp ip-address command.


I hope you found this useful! If you did, I’d love you to browse through my other posts. I write up random stuff during my networking studies that I had to Google, and hopefully you’ll find them interesting and helpful.

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