BGP Best Path

[Madhu]

Hello Team,

I have a question based on the below logs. 

We have 2 different ISPs connected to my switch. I have accepted the default route alone from the 2 different ISPs. 

If I do sh ip ro, I can see there are 2 available paths.. 

Question: Does it do load balaning? 

How both default routes are become in routing table ? 

Logs: 

Router#sh ip bgp summ
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
X.X.X.65 4 9299 34222 37662 13 0 0 3w2d 1
X.X.X.169 4 4775 68450 74645 13 0 0 3w2d 1

Router#sh ip bgp neighbors X.X.X.65 received-routes
Network Next Hop Metric LocPrf Weight Path
*m 0.0.0.0 X.X.X.65 0 9299 i
Total number of prefixes 1

Router#sh ip bgp neighbors X.X.X.169 received-routes
Network Next Hop Metric LocPrf Weight Path
*> 0.0.0.0 X.X.X.169 0 4775 i
Total number of prefixes 1

Router#sh ip ro 0.0.0.0
Routing entry for 0.0.0.0/0, supernet
Known via "bgp 38445", distance 20, metric 0, candidate default path
Tag 4775, type external
Routing Descriptor Blocks:
* X.X.X.65, from X.X.X.X, 3w3d ago
opaque_ptr 0x7F4BDEF413E0
Route metric is 0, traffic share count is 1
AS Hops 1
Route tag 4775
MPLS label: none
X.X.X.169, from X.X.X.X, 3w3d ago
opaque_ptr 0x7F4BDEF412A0
Route metric is 0, traffic share count is 1
AS Hops 1
Route tag 4775
MPLS label: none

[MHM Cisco World]

"" *m"" <<- this answer your Q

You config bgp with multipath 

That why two path inject into RIB

MHM

[Joseph W. Doherty]

"Question: Does it do load balaning?"

It should, using the usual ECMP rules (which generally discount actual link utilization).

"How both default routes are become in routing table ?"

Explained in @MHM Cisco World 's reply.

[shambhu.kumar]

There may be chances for asymmetric routing due to the same 2 prefixes are installed in routing table.

[Joseph W. Doherty]

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@shambhu.kumar wrote:

There may be chances for asymmetric routing due to the same 2 prefixes are installed in routing table.

---

True.

It might also arise, even if you insure only one path is used for egress, but make available multiple ingress paths.

Of course, there's asymmetric routing and then there's asymmetric routing.  The former might be considered return traffic not using the same path as the outbound traffic.  Generally often not a problem except for devices that often need to "see" both directions' traffic, such as firewalls.  The latter is when a flow's packets may take different paths.  This can be adverse for some traffic flows which is why many routing devices don't do packet-by-packet routing by default.  (Also why, if possible as an option, generally the recommendation is to NOT enable it - although, it does usually provide an almost perfect multi path egress utilization split.)

Across the Internet, there's a not uncommon chance flows between the two end points will use different paths, but again, usually an individual flow's packets will not take different paths, at least in one direction.

Often, as long as a flow's unidirectional packets don't take different paths, everything works just fine and you obtain the benefit of some additional bandwidth.

@shambhu.kumar as I see you have a CCDE certificate, did you have some specific cautions in mind, or did you just want to note asymmetric routing, possible across multiple paths, has the potential for possible asymmetric routing issues?

[shambhu.kumar]

Agree with you what you wrote, Just wanted to add,

Asymmetric routing having their own Advantages and drawback
Asymmetric routing allows for better load distribution across the network. This balanced approach can prevent congestion and ensure more efficient use of available bandwidth, leading to optimized network performance.

While routers typically handle asymmetric routing without issues, firewalls and other security devices often require symmetric traffic flows to function correctly. Asymmetric routing can disrupt these devices’ ability to track sessions, leading to dropped packets.
Because firewalls need to see both the incoming and outgoing packets of a connection to track its state correctly, their placement in the network is critical. If a firewall only sees one direction of the traffic due to asymmetric routing, it won’t be able to maintain the connection state accurately. This can lead to issues with traffic being blocked or misidentified.

[paul driver]

Hello

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 wrote:@Madhu
Question: Does it do load balaning? 

---

Not exactly no , Even though 2 entries for the same prefix are installed it the rib, its not guaranteed that the rtr will load balance by default, you may experience traffic being routed in/out on a single interface due to the cef hashing algorithm which is not exactly 50/50 and cef polarisation initiating.

[Joseph W. Doherty]

Hopefully, you'll not think @paul driver and I are providing two different answers regarding LB.

What Paul is describing is "usual ECMP rules" which is more (technically) load sharing rather than load balancing.

The behavior Paul describes is usually more apparent during short time intervals, where you may see a wide disparity between the loading of the two paths.  Over a longer term (such as 24 hours), again usually, you'll see a 50/50 split, but there's no guarantee it won't be 100/0.

As an aside, Cisco's PfR, can do dynamic LB (even on top of default routes); it also can direct flows to the best performing path, for flows!  It does even more, and if you order before midnight tonight . . . 

Seriously though, I've used OER/PfR, and I found it impressive technology.

One story I like to tell, once I activated it, across a multi private WAN clouds, one group that complained about its impact, was the group doing active SLA monitoring.  SLA no longer showed as many performance issues, between sites, because OER/PfR would "see" them first and route around them!