pete.reluctantgreenie.com

The full route table is getting pretty big, and a simple way to limit the amount of memory usage on your routers, is to limit the number of AS-PATHs that you accept. If you are connected to the Tier 1 providers, then most major networks will only be 3 or 4 AS’s away, so anything else you can just use a default route for.

If I pick up full routes, show bgp all summary looks like this

show bgp all summary

374859 network entries using 53979696 bytes of memory
374859 path entries using 19492668 bytes of memory
63037/63037 BGP path/bestpath attribute entries using 8320884 bytes of memory
56456 BGP AS-PATH entries using 2223352 bytes of memory

and the route table looks like

show ip route summary

Route Source    Networks    Subnets     Replicates  Overhead    Memory (bytes)
connected       0           8           0           448         1376
static          1           0           0           56          172
bgp 64513       144752      229965      0           20984152    64451324
  External: 374717 Internal: 0 Local: 0
internal        4380                                            15466904
Total           149133      229973      0           20984656    79919776

So if I am reading it right, BGP is using 83MB of memory and the route table is using 80MB

If I want to limit the number of paths, I can do that by setting an as-path access list with the right regex

ip as-path access-list 10 permit ^[0-9]+( [0-9]+)( [0-9]+)( [0-9]+)$
ip as-path access-list 10 deny .*

In this example, I am allow 4 AS hops. if I wanted three, I would just remove the last ( [0-9]+) entry.

To apply this access list, you need to add a bgp filter into your bgp config

 address-family ipv4 vrf blah
  neighbor 1.2.3.4 remote-as 1239
  neighbor 1.2.3.4 local-as 1790 no-prepend replace-as
  neighbor 1.2.3.4 version 4
  neighbor 1.2.3.4 send-community both
  neighbor 1.2.3.4 filter-list 10 in
  neighbor 1.2.3.4 activate
 exit-address-family

Now bgp all sum looks like this

show bgp all summary

109629 network entries using 15786576 bytes of memory
109629 path entries using 5700708 bytes of memory
21302/21302 BGP path/bestpath attribute entries using 2811864 bytes of memory
19510 BGP AS-PATH entries using 780400 bytes of memory

Thats about 30MB of memory, rather than 80MB on BGP, and if I look at the route table, its at 23MB rather than 80MB – we have saved about 100MB of memory.

If your going to do this, you need to make sure you do not have soft-reconfigure turned on. Before you turn it off, make sure you understand what it does first

If I turn on soft-reconfigure, even with the filter-list in place, the bgp summary looks like this again ( 83MB )

show bgp all summary

374735 network entries using 53961840 bytes of memory
374735 path entries using 19486220 bytes of memory
63026/21302 BGP path/bestpath attribute entries using 8319432 bytes of memory
56451 BGP AS-PATH entries using 2221352 bytes of memory

Even though the route table does not have all the routes ( 24MB )

show ip route summary

Route Source    Networks    Subnets     Replicates  Overhead    Memory (bytes)
connected       0           8           0           448         1376
static          1           0           0           56          172
bgp 64513       40138       69516       0           6140624     18860488
  External: 109654 Internal: 0 Local: 0
internal        1997                                            4796348
Total           42136       69524       0           6141128     23658384

So, you can save a 100MB of memory with a little tweak, but its a minimal gain really – a better solution is to purchase more ram for your router – its not that expensive.

Cheers
Pete