Answers to Chapter 11 Review Questions

Answers to Chapter 11 Review Questions

1:

What is the difference between routing and Layer 3 switching?

A:

In one sense, nothing . In another sense, the term routing implies that the forwarding is software-based where the term Layer 3 switching implies that hardware-based forwarding is used. In both cases, general-purpose CPUs are used to handle control plane functions (such as routing protocols and configuration).

2:

Can the router-on-a-stick approach to inter-VLAN routing also support inter-VLAN bridging?

A:

Yes. Simply configure a bridge-group on multiple subinterfaces. Example A-6 bridges protocols other than IP, IPX, and AppleTalk between VLANs 1 and 2.

Example A-6 Router-on-a-Stick Configuration That Routes IP, IPX, and AppleTalk but Bridges Other Protocols

   interface FastEthernet1/0   no ip address   !   interface FastEthernet1/0.1    encapsulation isl 1    ip address 10.1.1.1 255.255.255.0    bridge-group 1   !   interface FastEthernet1/0.2    encapsulation isl 2    ip address 10.1.2.1 255.255.255.0    ipx network 2    bridge-group 1   !   interface FastEthernet1/0.3    encapsulation isl 3    ip address 10.1.3.1 255.255.255.0    appletalk cable-range 300-310 304.101    appletalk zone ZonedOut    ipx network 3   !   bridge 1 protocol ieee 

3:

How can the RSM be especially useful in remote office designs?

A:

It can be fitted with WAN interfaces if you use the VIP adapter.

4:

What are the strengths of the RSM approach to Layer 3 switching?

A:

Its unique capability to both bridge and route traffic in the same platform. For example, it is much easier to mix lots of ports that both bridge and route IP traffic using the RSM (and MLS) than it is to use IRB on IOS-based devices. See the section "MLS versus 8500s" for more information.

5:

Does MLS eliminate the need for a router?

A:

No. Because MLS is a routing switch Layer 3 switching technique, it relies on caching information learned from the actions of a real router. The router must therefore be present to handle the first packet of every flow and perform the actual access list processing.

6:

Does MLS require a router that runs the router-based NetFlow mechanism?

A:

No. Other than the fact that MLS and NetFlow on the routers can both be used for detailed data collection, the two mechanisms are completely separate. A router doing MLS processing does not need to be running router NetFlow.

7:

In MLS, does the router create the shortcut entry and download it to the Layer 3 CAM table located in the Catalyst's NFFC or MSFC?

A:

No. Many people are of the opinion that MLS is simply a router running router NetFlow that learns a flow and then ships the results of this flow to a Catalyst. This is not the case. First, if it were the case, the flow would probably be over before the information could be learned by the Catalyst. Second, the NFFC learns the cache information totally by itself. It only needs to know the MAC address and VLAN information of the router (it learns this via MLSP).

8:

What is a flow mask?

A:

A flow mask is used to set the granularity with which MLS creates flows and builds shortcut entries. There are three flow masks: destination, source-destination, and full. See the section "Access Lists and Flow Masks" for more information.

9:

How does the Catalyst 8500 make routing decisions?

A:

It uses a general-purpose CPU to build a routing table and then a CEF table that gets downloaded to the line cards. The line cards use ASICs to perform lookups in the CEF table and make forwarding decisions. See the section "Switching Routers" for more information.

10:

What are the two routing options offered for the Catalyst 6000 family? From a conceptual standpoint, how do they differ?

A:

The MSM and the MSFC. The MSM is a switching router style of platform (it is based on 8510 technology). The MSFC uses MLS (however, it contains both the MLS-SE and MLS-RP on the same card).

11:

What is MHSRP? How is it useful?

A:

MHSRP stands for Multigroup Hot Standby Router Protocol. It is a technique that creates two (or more) shared IP addresses for the same IP subnet. It is most useful for load balancing default gateway traffic.

12:

What is the difference between CRB and IRB?

A:

Although both features allow a particular protocol to be routed and bridged on the same device, CRB does not let the bridged and routed halves communicate with each other. IRB solves this by introducing the BVI, a single routed interface that all of the bridged interfaces can use to communicate with routed interfaces in that device.

13:

When is IRB useful?

A:

When you want to have multiple interfaces assigned to the same IP subnet (or IPX network, AppleTalk cable range, and so on), but also want to have other interfaces that are on different IP subnets. The interfaces on the same subnet communicate through bridging. All of these interfaces as a group use routing to talk to the interfaces using separate subnets.

14:

What are some of the dangers associated with mixing bridging and routing?

A:

In a general sense, mixing the two technologies can lead to scalability problems. Specifically, it merges multiple Spanning Trees into a single tree. This can create Spanning Tree instability and defeat load balancing. It can lead to excessive broadcast radiation. It can make troubleshooting difficult. In general, it is advisable to create hard Layer 3 barriers in the network to avoid these issues.

15:

What is the benefit of using the IEEE and DEC Spanning-Tree Protocols at the same time? Where should each be run?

A:

Both protocols can be used to avoid the Broken Subnet Problem. IEEE must be run on the Layer 2 Catalysts (they only support this variation of the Spanning-Tree Protocol). The IOS-based routers therefore need to run the DEC or VLAN-Bridge versions.